Installation of a fuel cell in a vehicle has been practiced because the fuel cell has low environmental impact. In a fuel cell, for example, a fuel gas such as hydrogen is supplied to an anode side of a fuel cell stack, while an oxidation gas including oxygen such as, for example, air is supplied to a cathode side of the fuel cell stack, to thereby cause an electrochemical reaction through an electrolytic film and extract necessary power from the electrochemical reaction. Water is then produced by the electrochemical reaction on the cathode side. The produced water is contained in a spent oxidation gas on the cathode side and delivered to the outside. Further, the produced water penetrates the electrolytic film into the anode side, and the penetrated produced water is included in a spent fuel gas and discharged to the outside.
Although a solid polymer membrane, for example, is used as the electrolytic film, in some cases fluorine or the like, which is a constituent element of the solid polymer membrane, may be extracted into the produced water. As a result, highly corrosive ions such as fluorine ions are produced, which may sometimes cause corrosion of a gas pipe and the like.
For example, JP 2002-313404A addresses such a corrosion problem, and discloses that ions discharged from a solid polymer membrane are captured by providing a first ion removal unit in at least a pipe, among fuel gas discharge pipes and oxidizing gas discharge pipes, through which produced water is discharged, providing a second ion removal unit in a fuel gas humidifier and an oxidizing gas humidifier, providing a third ion removal unit in at least one of a fuel gas humidifier side of a cooling water feeding pipe and an oxidation gas humidifier side of a circulation pipe, providing a fourth ion removal unit in both a location in a mid-point of a cooling water bypass pipe and a location in a mid-point of a circulating water bypass pipe. Here, the ion removal units include ion-exchange resin.
Other than ion removal as described in JP 2002-313404A, corrosion resulting from fluorine ions or the like may be addressed by forming pipes using a material having high corrosion resistance such as, for example, stainless steel. However, because austenitic stainless steel contains, as one of its components, nickel, and the nickel may be, in some cases, eluted by the fluorine ions, it becomes necessary to suppress an amount of nickel emission in compliance with emission standards when the produced water is discharged. With this in mind, it is conceivable to use ferritic stainless steel which does not include nickel, such as JIS SUS 436 stainless steel, for example.
On the other hand, a gas pipe itself is welded and fixed to a fixing portion to ensure that the gas pipe is securely fixed so as not to cause gas leakage or the like. Although a brazing technique may be used for the welding, processing at a certain level of high temperatures is needed for such a welding technique. When the ferritic stainless steel is processed at high temperatures, Cr is combined with C at a grain boundary, thereby precipitating chromium carbide, and the content of Cr is accordingly reduced in the vicinity of the grain boundary. Therefore, high-temperature processing of ferritic stainless steel is known to cause the ferritic stainless steel to become sensitive to corrosion. Because it is likely that the occurrence of such sensitization will result in grain boundary corrosion, the fixing method which needs high-temperature processing has a problem to be solved.
As described above, it is difficult to ensure compatibility between high resistance to corrosion caused by fluorine ions contained in produced water and a method for fixing a gas pipe in a gas piping system for a fuel cell.
The present invention advantageously provides a gas piping system for a fuel cell in which both high corrosion resistance and pipe fixing robustness can be implemented.